STC-1000 thermostat retrofit to fridge freezer

[sparx]

Hi all!

I've an old fridge freezer that has been relegated to the laundry room, a single compressor, freezer on bottom type. The laundry room is unheated, and this fridge freezer has been misbehaving since. It draws a max of 120W, getting down to about 93-94W when the fridge and freezer are at temp, as I've been monitoring the cycling on a smart plug. Temperature swings in the fridge (measured with a Switchbot humidity/temp sensor) are 3-10 degrees C, and although I've not measured the freezer with the switchbot, I've had anywhere from -26 to -10C in the supposed coldest drawer with a digital thermometer. There isn't any "fast freeze" or "cold temperature override" function. Pulling the temperature sensing tube out of the fridge wall and leaving it inside makes the compressor run continuously and we get below zero fridge temps, so I think the compressor and gas is all ok.

I'm planning on running the temp controller off a separate mains/transformer (depending on type of unit I get), and then using some appropriately rated two core flex to join into the "always live" connection and switch into the "call for cooling" connection from the thermostat.

I'm unsure if these units keep their memory after a power outage, there isn't much documentation out there if they do or not. So I was considering leaving the existing thermostat and wiring in place, turning the temperature control to the "warmest", wiring the STC-1000 in parallel, and therefore always have a backup to keep the freezer food frozen in case the worst happened.

Are there any suggestions from the wise for this project? Would the 10A relay be suitable for switching the compressor in/out?

Thanks

[themadhippy]

I'm unsure if these units keep their memory after a power outage,

The elitech unit ive used for a similar project  certainly does.I didn't bother touching the fridge  wiring,just put it inline with the incoming supply , set the fridge stat to the coldest it would go and let the stc  control the temperature

[sparx]

I would like to retain the light though, hence thinking I could tap into the wiring by the compressor. I did consider putting the whole thing inside the fridge (and ripping out the existing thermostat housing and stuff) but again, the light... unless I wire up some LEDs and go full DIY on the thing. The current light switch integrates with a plastic thing with a metal core (4 wires come up, 1 live, 1 neutral, 1 earth and 1 switched live) which clips onto this plastic/metal thing, the light switch is as basic as can be and is a little metal rod that is moved against two metal pads...

Cheers!

[sparx]

Ordered one off Amazon and it got here pretty sharpish. I've got an old plug and length of 2x0.75mm flex from a small appliance I can use to power the thermostat. It does indeed keep it's memory, but only after you go back off the settings screen - if you stay on the settings screen it seems to forget everything if unplugged! It also differs to bigclive's video, there is no transformer, it has mains, directly through a diode on one of the legs, into a small SOIC8 (I'm getting old, I couldn't read the number). There is a good 10mm between mains in and the thermistor although there is no clearance cutout. I printed an enclosure for it, and made some holes for PG cord grips to fit.

My plan is to pull the fridge/freezer out, unscrew the black cowl over the connections, find the two brown wires that go up to the fridge thermostat/light and break them, using a couple of 3-way Wagos to connect everything back up. I've got a length of 2x1.5mm flex for the relay connection to the STC-1000. I believe this should mean the original thermostat will kick in if the STC-1000 for some reason does not (memory lost, or relay failure). The thermistor for now will snake through the door seal into the fridge, so I can figure out the best place for it, and once I've figured that out, I'll see if I can fish something down the cable route from the existing thermostat and pull it through there, or gingerly make a hole through the fridge body.

I also found a diagram for the Ranco thermostat adjustment screws! Success.... or so I thought. The one screw adjusts the "cut in" temp,  and the dial in the fridge appears to adjust the "cut out" temp. If you have too short of a range between the two, and the cut in too low, the fridge may stay cold, but the freezer starts to warm up! Interesting. I believe the thermostat also measures the cold plate in the fridge rather than the fridge temp itself, so that, accompanied by the low ambient temps, the fridge just stays too cold. Lots of videos online of people installing "winter heaters" and "garage heaters", or leaving the bulb on in the fridge..... but I also wonder if the end of the thermistor sense tube is actually in the right place, it's some 15 years old and it's been moved around a lot in its life.

[sparx]

STC-1000 fitted and working!

There is a bundle of cables from the compressor electronics box to up inside the fridge/freezer inside a rubber sleeve. I opened the sleeve (and the electronics box) to figure out which wire was which. There were spare spade terminals for every connection except the incoming live... So I had to join mid-way up the cable to the original thermostat. I used some Wago terminals which were on hand, I'll check how they're holding up in a short while as they're near the compressor.

I obtained a 12V version of the STC-1000 after deciding that I would use a higher current rated relay to switch the compressor on, so the STC-1000 switches 12V to a relay that sits in the base of the fridge freezer.

The thermistor location is still being worked out, although taped about 3 inches below the door switch seems to keep the fridge at a nice well controlled 1-3 degrees, and freezer at -20 to -24.... so result! Also has the nice added benefit that if the door remains open because you're putting stuff in from the supermarket, it kicks back on and starts cooling immediately, and the daily power usage has now returned to it's rated power of 1.44kW.

I thought I took pictures, but unless my phone has lost them, I might have been too far into the project to remember!